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Building is_prime() with TDD
First test a non-prime such as 4: expect False. The test fails; add a loop that returns False when num % i == 0, then it passes.
Next test is_prime(5): expect True. It fails; add return True and it passes. The test for 2 already passes.
The test for 1 fails; add if num == 1: return False. The test for 0 fails next; add the 0 check. Develop one small test at a time.